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Original question:

DRV8842 Operation Error

DRV8842: DRV8842 Fault

Balaji Padmanaban
Intellectual 645 points
Part Number: DRV8842
Other Parts Discussed in Thread: DRV8704

HI All,

Am having an issue with the HBRIDGE DRV8842 and the behavior is different in EVM and in my board.

The schematic is pretty much the same.

I have document my observation in the word document.

Can someone please help?

We are interfacing STM32F4 (MCU) with DRV8842PWPR

 

Motor:

AE200: 24VDC, Ampere Draw Max,

1.38Amps Average, Peak Starting Current 9A for 3.22ms

 

HBRIDGE Setting:

VREF: 1V

ISENSE: 0.1Ohms

ICHOPPING CURRENT: 2A(1/(5*0.1)

ICURRENT SETTING :(I4:I0)=11111

Sleep,Reset,IN1, IN2 ,I[4:0] controlled from MCU.

 

Power Supply:

The HBRIDGE is supplied by 24V Supply from board.

 

Schematic:

EVM Schematic:

Board Layout:

Bottom Layer:

 

Top Layer:

 

 

Issue:

The DRV8842 doesn’t drive the motor consistently and triggers the fault when OUT2 or OUT1 gives 24V at times.

 

The DRV8842 EVM works perfect with the motor and the one on my board doesn’t.

The schematics is pretty much similar with the EVM.

 

Normal Working when not driving motor:

Case 1(Fault Trigged):

 

But the above doesn’t happen in the EVM.

Is it because of the starting current of 9A for 3ms which is causing this?

 

Case2:

The HBRIDGE is not responding to the commands on IN1 & IN2 on OUT1 & OUT2.

 

Hint:

  1. I powered the EVM with my board’s 24V and the board works fine thought the 24V dips to 16V when starting the motor for 12ms.
  2. I have a bulk of 100uF 63V aluminum similar to EVm on board.
  3. I have the same sense resistor value of EVM (0.1Ohms as per EVM BOM)
  4.  
  5.  

Can I send the HBRIDGE to TI for RMA and dechiping it for analyzing?

 

Questions:

  1. What is the OCP response time and the Isense Response time (is ISENSE 3.7us?)
  2. How to limit the inrush current of the Motor if HBRIDGE doesn’t do it?
  3. Why the H Bridge allows to rise the current and doesn’t go to PWM mode when exceeding the chopping current?
  4. Why the EVM behaves the correct way?
  5.  

 

 DRV8842 Issue.docx

  • 0
    TI__Guru** 114905 points
    Hi Balaji,

    If you are operating above 6A, the overcurrent protection can activate. The overcurrent trip point varies from device to device so it is possible the EVM may work and your board may not. It is also possible that one direction operates while the other does not because of variation of the overcurrent trip point in the device.

    Please place a .1uF capacitor across the sense resistor. This will help.

    If you need to operate 9A for 3ms, please consider a different device. The DRV8704 with external FETs may be a good choice. I noticed you may have two DRV8842 on the board. If the current levels are the same, the DRV8704 can work if you have a SPI on board to communicate.
  • 0 in reply to Rick Duncan
    Intellectual 645 points

    Thank you Rick.

    I have placed a 0.1uF across the sense resistor to assist but the results were same.

    But the the AC cap across would short the transient (starting inrush)across the sense resistor to ground which is acting against my goal.

    I will consider DRV8704 for next revision .

    We have another board which has 8 of these HBRIDGES.

    Is there any method i limit the starting current using starters used in DC motors(5 pints,star delta for big motors).

    Am okay to add a series current limit to make it work for this revision.

    Would a inductor of number 530-HCTI-100-7.0 help me?

  • 0 in reply to Balaji Padmanaban
    TI__Guru** 114905 points
    Hi Balaji,

    If the chopping current is set to 2A, the current may be increasing during the blanking time and causing an overcurrent event.

    If this is the case, you should be able to monitor either the current or the voltage across the sense resistor to detect this overcurrent condition. Adding an inductor to limit the current may help, but please determine if the current is from the motor or a capacitor across the windings. If it is a capacitor, you may consider a different motor.

    Have you monitored the outputs to determine if the TVS diodes are affecting the output?
  • 0 in reply to Rick Duncan
    Intellectual 645 points
    Hi Rick,

    I thought the blanking time time is for when the IN1 & In2 or ISET changes state to control control current.
    Basically from input change to sense detect.
    My motor rises the current at 1.5m to 9A which is far greater than the blanking time.
    Am i correct here?

    I added a 0.1ohms in series with the motor return to see the current.(it was 0.9/0.1E)=9A.

    The motor doesn't have capacitor to produce phase shift for starting.
    it has a PTC fuse.

    I have also removed the TVS from the HBRIDGE output and same thing happens.
    My VM pin is with 24V+/-5% for load upto 6A.


    Am gonna try adding an induct for a frequency of (1.5/risetime) and calculate the impedance required for it.

    I will keep you updated on the results.

    Also your valuable ideas can also help me.
  • 0 in reply to Balaji Padmanaban
    TI__Guru** 114905 points
    Hi Balaji,

    Sorry for the delay.

    Would you please help me understand page 6?

    What is the timing between:
    RESET and SLEEP
    SLEEP and IN1
    IN1 and IN2

    Where do you see 9A in the scope capture?
    In the 6ms prior to the fault, is OUT1 really 6V and OUT2 0V?
    Can you change the timescale to 5us/div and move the trigger toward the end of the screen? This will help determine what caused the fault.
    Can you change the timescale to 50ms/div and move the trigger toward the start of the screen? This will help determine what type of fault occurred.
  • 0 in reply to Rick Duncan
    Intellectual 645 points

    Hi Rick,

    Adding to my above attempts,

    I added a series resistors to to  limit the starting current of the motor to 2A during the 9ms period and the motor doesnt stall.

    the average current for the motor seems to be 0.3A.

    I have tested with my motor for 12hours with 60times per minute.

    I feel it was the OCP which was tripping the fault due to the inrush.

    BR,

    Bala

  • 0 in reply to Balaji Padmanaban
    TI__Guru** 114905 points
    Hi Bala,

    The current chopping feature is designed to regulate the current during startup. This should eliminate the need to a series resistor.

    Would you also try:
    1) removing the 10k pulldown resistor on the DECAY? This will change the decay mode from slow to mixed and may limit the current further during startup.
    2) if mixed decay does not work, please try connecting the DECAY pin to 3V3 directly to set the decay mode to fast.
  • 0 in reply to Rick Duncan
    Intellectual 645 points
    Option 1 & 2 didn't help.
  • 0 in reply to Balaji Padmanaban
    Intellectual 645 points
    Hi Rick,
    Do you have TI rep who can come over here and see the actual issue?
  • 0 in reply to Balaji Padmanaban
    TI__Guru** 114905 points
    Hi Bala,

    Were you able to grab scope captures of the signals requested above?

    If so, can you attach them?
  • 0 in reply to Rick Duncan
    TI__Guru** 114905 points
    Hi Bala,

    I’m assuming you were able to resolve your issue. If not, just post a reply below (or create a new thread if the thread has locked due to time-out)